Highly purified water is required in many applications. Known purification apparatus usually comprises a housing which accommodates purification material such as ion exchange resins and reverse osmosis membranes. The housing includes an inlet by means of which water can be fed to the purification medium and an outlet through which the purified water can be extracted. In the known apparatus the outlet usually forms part of the main housing and is therefore located in very close proximity to the main housing in order to prevent recontamination of the purified water. Such apparatus has limitations since the outlet may not be easily accessible. Another problem with the known apparatus concerns the valve which controls flow through the outlet. The valve is usually designed to reduce re-contamination of the water to a minimum but this can lead to difficulties in achieving an accurate flow rate, thus making it difficult to extract precisely defined volumes of liquid.